1. Field of the Invention
This invention relates to a method and apparatus for controlling the interstand tension imparted to a workpiece being rolled by rolling stands of a tandem rolling mill.
2. Description of the Prior Art
In tandem rolling mills, various rolling conditions must be maintained constant throughout the rolling operation in order that a workpiece can be rolled into a product having a uniform thickness, width and shape between the leading and trailing end portions thereof.
It is the variation in the interstand tension which exerts a serious adverse effect on the thickness, width and shape of the product, and it is therefore essential for the purpose of stable rolling to control this interstand tension to be constant throughout the rolling operation.
In a hot rolling mill in which a workpiece is heated up to a high temperature to facilitate plastic working, a slight variation in the interstand tension exerts a great adverse effect on the dimensions and quality of the rolled product. Further, this variation in the interstand tension gives rise to troubles including severing of the workpiece being rolled.
In order to ensure the stable rolling operation, therefore, the rolling equipment is required to include suitable interstand tension control means. For example, in a hot finishing rolling mill for applying finishing rolling to a workpiece, a mechanical interstand tension control means called a looper is provided. The manner of interstand tension control using this looper will be described below by way of example. When the leading end portion of a workpiece is fed into the nip between the rolls of an (i+1)th rolling stand after passing through an i-th rolling stand of the rolling mill, the looper disposed between these rolling stands is set up to form a loop of the workpiece and maintains the loop until the end of the rolling operation so as to prevent impartation of an excessively high tension to the workpiece. However, the prior art system employing such a looper involves the problem that an excessively large interstand tension is imparted to the workpiece at the time of the initial setting up of the looper resulting in a reduction in the precision of the thickness of the workpiece being rolled. Further, in this prior art system, various kinds of disturbance encountered during rolling, for example, the presence of thermal rundown and skid marks in the longitudinal direction of the workpiece tend to give rise to instable rolling operation resulting in impartation of an excessively large tension or damage to the workpiece. Further, it is difficult to ensure the required performance of the looper since the looper is placed in an environment in which a very high temperature and much moisture prevails. Furthermore, such a system is only applicable to hot rolling of a workpiece into a strip and is not applicable to rolling of a workpiece into an angle bar, a round bar or the like.
In an effort to solve such problems, a method has been proposed in which the interstand tension is controlled by detecting it electrically without any mechanical contact with a workpiece. For example, U.S. Pat. No. 3,940,960 granted on U.S. Pat. application Ser. No. 541,953 filed January 17, 1975 and issued Mar. 2, 1976 discloses an interstand tension control based on the ratio between the rolling torque and the rolling force. The operation of the apparatus disclosed in the U.S. patent will be briefly described. The rolling force P.sub.10 and rolling torque G.sub.10 at a first rolling stand are detected after a workpiece is fed into the nip between the rolls of the first rolling stand but before the workpiece is fed into the nip between the rolls of a next adjacent second rolling stand, and the ratio G.sub.10 /P.sub.10 therebetween is stored in a memory. This ratio G.sub.10 /P.sub.10 represents the torque arm for the first rolling stand in the state in which the workpiece at the outlet of the first rolling stand is tension-free. Then, the rolling forces P.sub.1B , P.sub.2B and rolling torques G.sub.1B, G.sub.2B at the first and second rolling stands, immediately after the workpiece is fed into the nip between the rolls of the second rolling stand, are detected, and the torque arm G.sub.20 /P.sub.20 for the second rolling stand in a tension-free state is computed on the basis of these detected values. Then, the rolling speed of the first or second rolling stand is controlled so that (G.sub.10 /P.sub.10) - (G.sub.1 /P.sub.1) representing the difference between the torque arm value G.sub.1 /P.sub.1 detected at the first rolling stand during the rolling operation and the torque arm value G.sub.10 /P.sub.10 stored in the memory, hence, the torque arm variation becomes equal to (G.sub.20 /P.sub.20) - (G.sub.2 /P.sub.2) representing the difference between the torque arm value G.sub.2 /P.sub.2 detected at the second rolling stand during the rolling operation and the torque arm value G.sub.20 /P.sub.20 stored in the memory, whereby the interstand tension imparted to the workpiece can be controlled to be constant throughout the rolling operation.
Such a manner of electrical tension control is adopted in angle bar or round bar rolling mills and rough hot rolling mills and contributes greatly to the realization of the desired stable rolling operation. In these rolling mills, automatic adjustment of the roll gap for the control of the dimensions of products is not carried out in many cases. On the other hand, in a hot finishing rolling mill, the roll gap is positively adjusted or varied so as to control the workpeice thickness with high precision. It has been found that the interstand tension tends to vary in the hot finishing rolling mill when the aforementioned method, in which the variation of the torque arm value at the first rolling stand is controlled to be equal to that of the torque arm value at the second rolling stand, is applied directly for the interstand tension control in the hot finishing rolling mill. Therefore, this technique is not suitable for direct application to the hot rolling mill in which the roll gap must be positively varied. Especially, in the hot finishing rolling mill, such a slight variation in the interstand tension exerts a considerably serious adverse effect on the product since the thickness of the workpiece is quite small.